Measuring instruments or tools are commonly lowered down a wellbore and located in a subterranean reservoir to measure formation characteristics such as bottom hole pressures and temperatures as a function of time. These instruments are available in "memory" type and "real-time" type configurations.
The "memory" type tools are conventionally lowered through a tubing string positioned in a wellbore on a 1/8" diameter solid wire called a wireline. This economical and successful procedure and its associated equipment is known as a slick line system and is used mainly for bottom hole pressure testing. These "memory" type tools remain downhole recording data onto a downhole chart for a finite time period. The tool is then retrieved to ascertain the measured data accumulated over the entire period.
The slick line system is popular due to its minimum equipment requirements and its ease of use.
Applied to a known well or reservoir, the operator of the slick line system can usually estimate the length of time required for the tool to remain downhole and obtain the desired data. If, however, the tool is retrieved prematurely, the data could be incomplete and therefore of questionable value. Conversely, valuable time is lost and needless expense incurred if the tool is left downhole longer than necessary. For example, in the case of a pressure drawdown/build-up test, the tool may be left in the well for a number of days with the expectation that the pressure build-up will have stabilized by the time the tool is recovered--however stabilization may not yet have been achieved.
To date, the slick line systems have been limited to use with "memory" type tools, as a compatible wireline has not been available for transmitting signals generated by "real-time" type logging tools to the surface.
"Real-time" type tools both avoid the time uncertainties of the "memory" type tools and supply the operators with data at the surface as it is generated. Signals are generated at the reservoir-located tool and are transmitted up a conductive wireline to a surface-located data acquisition system. The wireline is generally comprised of a single conductor and is implemented a variety of forms requiring various degrees of specialized equipment not in common with the slick line system.
One form of the conductive wireline is a 1/4" tube within which is located a single, solid electrical conductor. The conductor is typically bonded within the bore of the tubing with epoxy or teflon, thereby insulating the conductor therefrom. The tubing extends through the wellhead and into the wellbore. The tubing is strapped to the outside of the tubing string and extends downwards to the tool which is typically mounted to the bottom of the tubing string. The bonded tube, conductor and epoxy insulation are unforgiving with respect to flexing or thermal exposure and are particularly prone to damage.
Flexing occurs whenever the 1/4" conductive wireline is installed or removed. The wireline is usually spooled onto a large diameter (&gt;6 foot diameter) drum. When coiled, each of the tubing, the insulation, and the conductor components are laying at differing physical lengths of circumference. This varying pitch results in differential movement of each component and repeated deformation. Similarly, differential thermal expansion and relative movement occurs when the tubing changes temperature with respect to the conductor. Multiple use and re-use of the wireline results in eventual failure of the conductor, the insulation or both, resulting in permanent damage to the wireline. For these reasons, handling of the tubing, or differential thermal expansion experienced in the wellbore often result in irreparable damage to the expensive assembly.
An alternate arrangement for a "real-time" tubing and conductor assembly is as described in U.S. Pat. No. 5,122,209, issued to Moore. A plurality of insulated conductors are helically coiled and simultaneously installed within 1/4" tubing during its manufacture. Thus the tubing and the conductors are not bonded together to form a solid unit. This arrangement ameliorates much of the differential movement difficulties of the previous bonded tubing/conductor/epoxy assembly. Friction between the conductor's helical coils and the tubing restricts free movement within the tubing, yet the conductors can extend, contract and shift to accommodate variable pitch when spooled and to relieve some of the thermal effects. The tool and wireline are again secured to the outside surface of the tubing string, which must be tripped out of the well to recover the tool.
Both of the 1/4" tubing and conductor wirelines are generally used in permanent installations. Each of them require tripping of the tubing string to repair or change the tool. The production delay or suspension of a well operation to trip tubing from a well can be expensive. Further, a rig is required at the well to effect the removal of the tubing string, Still further, significantly larger scale wireline spool and associated equipment is required, than is used in the slick line system.
In yet another form of wireline, one or more insulated conductors are contained within a flexible and conductive braided outer sheath. The braided wireline and an attached tool can be run into and removed from the bore of a tubing string without requiring the removal of the tubing string. Braided wirelines of diameters less than 3/16" (for one conductor) are not known to the inventor.
In the braided wireline embodiment, the tool is introduced to the tubing string through an isolating lubricator mounted at the wellhead. The braided wireline requires significant effort and specialized equipment to seal it at the lubricator. Typically an extra person is required, dedicated to managing a pressurized grease sealing system. The braiding conforms in length to the conductor length, being more robust and suited to multiple use than are the 1/4" tubing and conductor assemblies. The braiding is expensive however, and due to the high surface area and the braid's tendency for corrosion, expensive and exotic materials are required when used for sour (H.sub.2 S) service. The need for increased labor and specialized equipment and the very high unit length cost of the braided conductors limits use of this system primarily to production logging.
In summary, none of the prior art wirelines are amenable to being used with existing slick line equipment in "real-time" testing applications. The braided wireline is generally too expensive for use in other than production logging and requires specialized equipment not available with the existing slick line systems. The 1/4" wirelines suffer the following incompatibilities for use with the slick line system: the wireline diameter is too large and too inflexible for spooling onto conventional slick line drums; the wireline is too inflexible to be drawn down into the well under the weight of the attached tools alone, therefore requiring specialized draw-works; and the wireline requires a larger guiding sheave and packing seal at the lubricator.
An object of the present invention, therefore, is to provide a conductive wireline amenable for use with conventional slick line equipment currently in popular and widespread use, for obtaining "real-time" surface readout for both bottom hole pressure testing and production logging applications.